Voltage controlled oscillating circuits are used as local oscillation sources in tuners for satellite broadcasts or tuners for cable television broadcasts. The voltage controlled oscillating circuit includes a resonant circuit having a variable capacitance device, and a negative resistance circuit section having an active component. The voltage controlled oscillating circuit oscillates a predetermined frequency (resonance frequency) according to a controlled voltage applied to the variable capacitance device.
One indicator for the performance capabilities of the voltage controlled oscillating circuit is phase noise (unwanted energy generated in the vicinity of output frequency). The phase noise is affected by Q of the resonant circuit. That is, the higher Q the resonant circuit has, the greater difference is provided between the signal level and the noise level, enabling reduction in the phase noise.
FIGS. 18 and 19 illustrate exemplary structures of a conventional inductor, which has multilevel wiring and is used in the resonant circuit. FIG. 18 is a top view of the conventional inductor, and FIG. 19 is a cross-sectional view illustrating the inductor taken along line A-A′ of FIG. 18.
As illustrated in FIGS. 18 and 19, an inductor 121 includes a semiconductor substrate 106, an SiO2 film (not shown) formed on the semiconductor substrate 106, a plurality of insulating layers 108, a linear wire 103, a spiral wire 102, and a through hole 115.
The insulating layers 108 are stacked on the semiconductor substrate 106, and the SiO2 film intervenes therebetween. The spiral wire 102 is formed on an insulating layer 116, i.e., an uppermost layer of the insulating layers 108. The linear wire 103 is formed on another insulating layer (an interlayer, i.e., the second insulating layer from the uppermost layer) 117. The through hole 115 serves to establish electric connection between the wire 102 and the wire 103.
As illustrated in FIG. 18, a terminal 114 of the inductor 121 is provided on leading end of the wire 102, and connected to a terminal or the like of an external circuit (not shown). Further, another terminal 113 of the inductor 121 is provided on leading end of the linear wire 103, and connected to a terminal or the like of an external circuit (not shown).
In the spiral wire 102, a resistance (wiring resistance) occurs which is in series to the wire. This will be a major cause for reduction in Q of the inductor 121, thereby reducing Q of the resonant circuit incorporating the inductor. As such, a reduction in Q of the resonant circuit causes an increase in the phase noise.
Note that, related technical art is disclosed in Japanese Unexamined Patent Publication, No. 68862/2003 (Tokukai 2003-68862, publication date: Mar. 7, 2003) and Japanese Unexamined Patent Publication, No. 97377/1996 (Tokukaihei 8-97377, publication date: Apr. 12, 1996).
As a solution for the above problem, the resistance can be reduced by making the wires of the spiral wiring to be broad in width. However, this results in increasing the area of the inductor, and further increasing the area of the circuit.
As another solution, the resistance can be reduced by forming a thick spiral wire on the uppermost insulating layer. However, this requires a special thick insulating layer, used exclusively for an inductor, to be formed on the uppermost insulating layer.